Asthma: Definition of severe cases and use of high dose inhaled steroids
Information from NICE's Covid-19 rapid guideline: severe asthma
Severe asthma definition
In rapid guidance produced by NICE in light of Covid-19, it says -
Be aware that severe asthma is defined by the European Respiratory Society and American Thoracic Society as asthma that requires treatment with high-dose inhaled corticosteroids (see inhaled corticosteroid doses for NICE's asthma guideline) plus a second controller (and/or systemic corticosteroids) to prevent it from becoming 'uncontrolled', or which remains 'uncontrolled' despite this therapy.
Source: NICE, Covid-19 rapid guideline: severe asthma NICE guideline [NG166] [published 3 April]